There are known technologies for generating image data representing a combined image formed by joining a first image with a second image. For example, one image processing device known in the art uses a scanner to perform two passes over an original that is too large to read in one pass, acquiring scan data representing a first image and scan data representing a second image. On the basis of two sets of scan data, the conventional image processing device generates image data representing a combined image formed by combining the first and second images. In this case, a pattern matching is used to determine a joining position at which the first and second images are combined.